Process for preparing ester fluorinated ion exchange polymer precursor by acid treatment of ether

ABSTRACT

Vinyl ether monomers of the formula ##STR1## wherein n is 0 or 1 and R is CH 3  or C 2  H 5 , precursors thereto, and copolymers with, e.g., tetrafluoroethylene, are provided. The copolymers are useful as electrical insulation and as the sheath portion of optical fibers, and can be converted to known ion exchange polymers having carboxylic groups which are useful, e.g., in the form of permionic membrane for separating the compartments of a chloralkali cell. The vinyl ether monomers can also be converted to vinyl ether monomers which contain carboxylate functional groups, which in turn can be copolymerized to useful copolymers.

BACKGROUND OF THE INVENTION

Fluorinated vinyl monomers have proved to be useful intermediates formaking highly fluorinated and perfluorinated polymers and copolymerswhich are useful, e.g., as electrical insulation, permselectivemembranes, and the sheath (cladding) layer of optical fibers.

It is an object of this invention to provide novel fluorinated vinylether monomers, precursors thereto, and methods for making same.

It is another object of this invention to provide novel highlyfluorinated copolymers containing ether linkages.

It is yet another object to provide new and improved methods for makingcertain known fluorinated vinyl ethers which contain carboxylatefunctional groups.

It is yet a further object to provide a novel method for making knownhighly fluorinated and perfluorinated ion exchange polymers whichcontain carboxylate functional groups.

SUMMARY OF THE INVENTION

According to the present invention, there are provided a chemicalcompound having the structural formula ##STR2## wherein n is 0 or 1 andR is CH₃ or C₂ H₅, copolymers thereof, and precursors thereto.

There are also provided, according to the present invention, improvedprocesses for preparing certain fluorinated vinyl ether monomers whichcontain carboxylate functional groups.

There is further provided, according to the present invention, a newmethod for making some fluorinated ion-exchange polymers which containcarboxylate functional groups.

DETAILED DESCRIPTION OF THE INVENTION

The vinyl monomers of the invention can be prepared by a series of stepsstarting with the known methyl 3-methoxytetrafluoropropionate (see,e.g., U.S. Pat. No. 2,988,537), 3-methoxytetrafluoropropionic acid, or3-methoxytetrafluoropropionyl fluoride (see, e.g., U.S. Pat. No.3,113,967). If the free carboxylic acid is used as the starting point,it is first transformed to the acyl fluoride; this can be done, e.g.,(1) in two steps by (a) reacting the free acid with any of a variety ofreagents such as PCl₅, POCl₃, SOCl₂ or benzoyl chloride at almost anypressure at a temperature of 25° to 250° C. to make3-methoxytetrafluoropropionyl chloride and (b) reacting the latter withany of a variety of reagents such as alkali metal fluorides or SbF₃ withor without a solvent at almost any pressure at a temperature of 50° to400° C., or (2) in one step by reacting the acid with SF₄ at roomtemperature and autogenous pressure. If the ester is used as thestarting point, it is first hydrolyzed to the free carboxylic acid, forexample by hydrolysis with acid or base. The acyl fluoride is alsodirectly available by reaction of methyl trifluorovinyl ether andcarbonyl fluoride (see J. Amer. Chem. Soc. 84, 4275 (1962)).

The immediate precursors of the vinyl monomers of the invention areprepared by reacting 3-methoxytetrafluoropropionyl fluoride withhexafluoropropylene oxide (HFPO). The reaction is carried out in thepresence of fluoride ion catalyst and a reaction medium.

The fluoride ion catalyst is provided by a fluoride compound whichdissolves in the reaction medium to the extent of at least 0.001% byweight at 20° C. Suitable fluoride compounds are potassium, rubidium andcesium fluorides. A preferred fluoride compound is potassium fluoride,as its use results in higher yields of the desired product. The fluoridecompound can be used in amounts of about 0.01 to 10 equivalents,preferably about 0.05 to 0.5 equivalent, per mole of3-methoxytetrafluoropropionyl fluoride employed.

The reaction medium can be an aprotic liquid in which the fluoridecatalyst is soluble to the extent of at least 0.001% by wt. at 20° C.(component A). Suitable examples include the so-called glymes (mono-,di-, tri- and tetraethyleneglycol dimethyl ether); lactones such as4-butyrolactone, 5-valerolactone and 6-caprolactone, and mononitrilessuch as acetonitrile and propionitrile. Triglyme and tetraglyme arepreferred because they are more easily separated from the product.

The reaction medium can also be, and preferably is, a mixture of 2 to50% by volume of component A and 98 to 50% by volume of a second aproticliquid (component B). Suitable examples of component B includedinitriles such as malono-, succino-, glutaro-, adipo-, methylmalono-,pimelo-, subero-, and phthalo-nitrile; and tetramethylenesulfone. Thedinitriles are preferred, and adiponitrile is especially preferred. Morepreferably, component A constitutes 85 to 98% by volume of the medium,and component B is 15 to 2% by volume. Most preferably, component Aconstitutes 85 to 95% by volume of the medium, and component B is 5 to15% by volume.

The reaction of 3-methoxytetrafluoropropionyl fluoride with HFPO isexothermic. Reaction temperatures can range from about 0° to 100° C.,with temperatures between 25° and 70° C. being preferred. Pressure isnot critical, and subatmospheric and superatmospheric pressures areoperable; pressures close to atmospheric are preferred. The pressure inthe reaction vessel can be controlled by regulating the rate of supplyof gaseous HFPO.

The precursor compounds so made have the structural formula ##STR3##where R is CH₃. When 3-methoxytetrafluoropropionyl fluoride reacts with1 equivalent HFPO, the precursor compound so made has the indicatedstructure where n is 0. That precursor compound can in turn react with asecond equivalent of HFPO to make the precursor compound where n=1.Small amounts of products wherein more units of HFPO are incorporatedare usually also formed. The relative amounts of the precursor compoundswhere n=0 and n=1 so made can be controlled by controlling the number ofequivalents of HFPO used as reactant; relatively lesser amounts of HFPOfavor formation of the precursor compound where n=0, and relativelylarger amounts of HFPO favor formation of the precursor compound wheren=1. If the precursor compounds are made by reacting HFPO with3-ethoxytetrafluoropropionyl fluoride, the precursor compounds have theindicated structural formula where R is C₂ H₅.

Such precursor compound is then subjected to a dehalocarbonylationreaction, wherein the elements of COF₂ are removed to produce the novelvinyl monomers ##STR4## where n is 0 or 1 and R is CH₃ or C₂ H₅. Thisreaction is suitably carried out by contacting the precursor compoundwith at least one member of the group consisting of Na₃ PO₄ and Na₂ CO₃at a temperature of at least 170° C., preferably 190° to 260° C.

These vinyl ether monomers can be copolymerized with other fluorinatedmonomers to make novel copolymers. Suitable comonomers include CX₂ ═CX₂where the four X's are four fluorines or three fluorines and onechlorine. Such copolymers comprise about 70 to 95 mol % --CX₂ --CX₂ --units where the four X's are as defined above, and about 5 to 30 mol %of substituted ethylene units of the formula ##STR5## wherein n is 0 or1 and R is CH₃ or C₂ H₅, the substituted ethylene units being randomlypositioned throughout the copolymer chain. The copolymers wherein thefour X's are four fluorines are preferred. These copolymers are useful,e.g., as insulation on electrical conductors, base for printed circuits,and as the sheath (cladding) portion of optical fibers.

The copolymers can be prepared by general polymerization techniquesdeveloped for homo- and copolymerizations of fluorinated ethylenes,particularly those employed for tetrafluoroethylene which are describedin the literature. Nonaqueous techniques for preparing the copolymersinclude that of U.S. Pat. No. 3,041,317, that is, by the polymerizationof a mixture of the major monomer therein, such as tetrafluoroethylene,and the fluorinated vinyl ether monomer in the presence of a freeradical initiator, preferably a peroxydicarbonate, a perfluorocarbonperoxide or azo compound, at a temperature in the range 0°-200° C. andat pressures in the range of 10⁵ to 2×10⁷ pascals (1-200 Atm.) orhigher. The nonaqueous polymerization may, if desired, be carried out inthe presence of a fluorinated solvent. Suitable fluorinated solvents areinert, liquid, perfluorinated hydrocarbons, such asperfluoromethylcyclohexane, perfluorodimethylcyclobutane,perfluorooctane, perfluorobenzene and the like, and inert, liquidchlorofluorocarbons such as 1,1,2-trichloro-1,2-2-trifluoroethane, andthe like.

Aqueous techniques can also be used for preparing the copolymer, andinclude contacting the monomers with an aqueous medium containing afree-radical initiator to obtain a slurry of polymer particles innon-water-wet or granular form, as disclosed in U.S. Pat. No. 2,393,967,or contacting the monomers with an aqueous medium containing both afree-radical initiator and a telogenically inactive dispersing agent, toobtain an aqueous colloidal dispersion of polymer particles, andcoagulating the dispersion, as disclosed, for example, in U.S. Pat. No.2,559,752 and U.S. Pat. No. 2,593,583.

The above copolymers can, if desired, be converted to esters of knownfluorinated ion-exchange polymers by treatment with a strong acid at atemperature of at least 50° C. but below the decomposition temperatureof the above-described copolymers, the product ion-exchange polymers,and the strong acid. The strong acids which are suitable for treatmentof the above copolymers to make fluorinated ion-exchange copolymers orprecursors thereto are suitably, e.g., H₂ SO₄, ClSO₃ H, FSO₃ H or R_(f)SO₃ H where R_(f) is a perfluorinated C₁ to C₈ group, or Lewis acids inwhich the halide is fluoride such as SbF₅. Temperatures of 80° to 150°C. are preferred. Such treatment of the above copolymers givescopolymers comprising about 70 to 95 mol % --CX₂ --CX₂ -- units whereinthe four X's are four fluorines or three fluorines and one chlorine, andabout 5 to 30 mol % of substituted ethylene units of the formula##STR6## where n is 0 or 1, Z is F or OR', and R' is at least one memberof the group consisting of R and H, the substituted ethylene units beingrandomly positioned throughout the copolymer chain. The carboxylic esterpolymers can be hydrolyzed to known carboxylic acid polymers which areuseful for ion-exchange purposes. Some hydrolysis of the ester polymersmay occur to varying degree during the treatment of the ether-containingpolymers with strong acid, the amount of hydrolysis varying with theacid and conditions used.

Such fluorinated polymers which contain carboxylic acid functionalgroups can be employed in various known ion-exchange uses. One such useis in the form of a permselective membrane for separating the anode andcathode compartments of a chloralkali electrolysis cell; theion-exchange capacity of the polymer for such use should be in the rangeof 0.7 to 1.5 meq/g (milliequivalents/gram), preferably 0.8 to 1.3meq/g.

The vinyl ether monomers ##STR7## or their bromine adducts ##STR8##i.e., compounds of the formula ##STR9## wherein Y is CF₂ ═CF-- or CF₂BrCFBr--, can also be converted respectively to vinyl ether monomerswhich contain carboxylic ester functional groups, having the structuralformula ##STR10## or their bromine adducts ##STR11## i.e., compounds ofthe formula ##STR12## wherein Y is CF₂ ═CF-- or CF₂ BrCFBr--, n is 0 or1, and R is CH₃ or C₂ H₅. When Y is CF₂ ═CF--, this conversion issuitably carried out by treatment with a strong acid at a temperature ofat least 25° C., but below the decomposition temperatures of both thestarting vinyl ether monomer and the product vinyl carboxylic estermonomer and the strong acid, preferably at 70° to 100° C.; above about100° C., some decomposition of the vinyl ether compound may occur. WhenY is CF₂ BrCFBr--, the conversion is suitably carried out by treatmentwith a strong acid at a temperature of at least 25° C., but below thedecomposition temperatures of both the starting brominated ethercompound and the product brominated carboxylic ester compound and thestrong acid, preferably at 100° to 150° C. The strong acids which aresuitable for treatment of the vinyl ether monomers or their bromineadducts to make compounds containing carboxylic ester functional groupsare suitably, e.g., H₂ SO₄, ClSO₃ H, FSO₃ H or R_(f) SO₃ H where R_(f)is a perfluorinated C₁ to C₈ group. The resulting vinyl monomerscontaining carboxylic ester groups can be copolymerized with otherfluorinated ethylenically unsaturated monomers, such as CX₂ ═CX₂ where Xis as defined hereinabove, to provide copolymers which can be hydrolyzedto the known fluorinated carboxylic acid ion exchange polymers referredto above.

The bromine adducts are suitably made by reaction of bromine with thevinyl ether monomers. Addition of bromine to the olefinic bond isfacilitated by irradiation with ultraviolet and/or visible light, asfrom a commercially available sun lamp. An inert solvent can be used butis not necessary.

The bromine adduct of the vinyl ester monomer which contains acarboxylic ester functional group can suitably be debrominated to thevinyl ether monomer which contains a carboxylic ester functional groupby, e.g., treatment with zinc.

Preparation of the vinyl ether monomer which contains a carboxylic esterfunctional group via the three step route of brominating the vinyl ethermonomer, acid treatment to convert the --CF₂ OR moiety to the --COORgroup, and debromination, is particularly advantageous because thebrominated compound is more thermally stable than the vinyl ethercompound.

Although that vinyl monomer with carboxylic ester group, referred to inthe previous paragraph, where n is 0 and R is CH₃, i.e., ##STR13## is aknown compound, the method disclosed herein for making it according tothe present invention is superior to a known method, thedehalocarbonylation of ##STR14## due to a sequence of reactions in theknown method starting with cyclization to form ##STR15## which knownmethod yields but little of the desired vinyl carboxylic ester monomer.It is also superior to another known method which starts withepoxidation of 1,1,2,3,3-pentafluoro-3-chloropropene-1. The methoddisclosed herein, ending with treatment of the vinyl ether monomer withstrong acid, provides an overall yield of about 50% for the four stepsstarting from

To further illustrate the innovative aspects of the present invention,the following examples are provided.

EXAMPLES

All temperatures specified herein are in °C.

Example 1 A. Preparation of CH₃ OCF₂ CF₂ COOH(3-Methoxytetrafluoropropionic Acid)

A mixture of 32 g sodium hydroxide, 400 g water and 152 g methyl3-methoxytetrafluoropropionate was stirred at room temperature until asingle liquid layer was obtained. The product was acidified with 37%aqueous HCl and the lower layer separated. The aqueous layer wasextracted four times with 50 ml ethyl ether and the combined etherextracts and lower layer distilled to give 103.3 g (73.4%)3-methoxytetrafluoropropionic acid, b.p. 85°-86° at 20 mm.

B. Preparation of CH₃ OCF₂ CF₂ COCl (3-MethoxytetrafluoropropionylChloride)

A mixture of 47.4 g 3-methoxytetrafluoropropionic acid and 67.3 gphosphorous pentachloride was heated and the contents distilled toobtain a pale yellow liquid boiling to 102°. Redistillation of thisliquid yielded 49.8 g (95.2%) 3-methoxytetrafluoropropionyl chloride,b.p. 84°-86°.

C. Preparation of CH₃ OCF₂ CF₂ COF (3-MethoxytetrafluoropropionylFluoride)

A mixture of 34.8 g potassium fluoride, 100 ml tetramethylene sulfoneand 49.8 g 3-methoxytetrafluoropropionyl chloride was slowly heated togive 35.4 g of colorless liquid (77.8%) whose infrared spectrum wasidentical to 3-methoxytetrafluoropropionyl fluoride.

D. Preparation of ##STR17## where n=0 and 1.(2-(3-Methoxyhexafluoropropoxy)tetrafluoropropionyl fluoride and

2-[2-(3-Methoxyhexafluoropropoxy)hexafluoropropoxy]tetrafluoropropionylfluoride)

A mixture of 1.5 g potassium fluoride, 56 g of a 9/1 volume/volumemixture of adiponitrile and tetraglyme and 44.3 g of3-methoxytetrafluoropropionyl fluoride were reacted at 30° with 44 g ofhexafluoropropylene oxide. The lower layer of the reaction mixture wasseparated and distilled to give 28.6 g (49%) of the product where n=0,b.p 50° at 100 mm, and 23.0 g (26%) of the product where n=1, b.p. 90°at 100 mm.

Example 2 Preparation of CH₃ OCF₂ CF₂ CF₂ OCF═CF₂(3-Methoxyhexafluoropropyltrifluoroethenyl ether)

A glass tube (2.5 cm diameter) packed with 125 g of dry trisodiumphosphate was heated to 225° and 30.4 g2-(3-methoxyhexafluoropropoxy)tetrafluoropropionyl fluoride was passedthrough it at a rate of 0.48 ml per minute. The crude product wasdistilled to give 17.0 g 3-methoxyhexafluoropropyltrifluoroethenylether, b.p. 54° at 200 mm, whose structure was consistent with itsinfrared spectrum and ¹ H and ¹⁹ F NMR spectra.

Example 3

Preparation of ##STR18##

A tube containing 125 g of dry trisodium phosphate was heated to 225°and 19.8 g ##STR19## added at a rate of 0.48 ml per minute. The crudeproduct was distilled to give 10.8 g ##STR20## b.p. 80°-82° at 100 mm,whose structure was consistent with its infrared and ¹⁹ F NMR spectra.

For purposes of further confirming the structure of the product, a smallportion of the above vinyl ether was reacted with excess bromine underirradiation of a "GE Sun Lamp." The crude product was washed withaqueous sodium bisulfite and distilled to give ##STR21## b.p. 196°,whose structure was consistent with its infrared and ¹⁹ F NMR spectra.

Example 4

Preparation of ##STR22##

A mixture of 10.0 g ##STR23## and 10.8 g 96% sulfuric acid was stirredat room temperature for 16 hours. The mixture was added to 100 ml water,and 9.3 g of a lower layer, almost all starting material, was recovered.The 9.3 g recovered material was heated at 80° for 16 hours with 15 ml96% sulfuric acid and the mixture added to 50 ml water to give 7.8 gproduct. Gas chromatographic analysis showed the product to contain 74%starting material and 21% ##STR24## For purposes of confirming thestructure of the product, a portion of the product was brominated togive material containing ##STR25## The gas chromatographic retentiontimes of these products were identical to those of authentic samples.The IR spectrum of the mixture showed an absorption at 5.6 microns(COOCH₃). The ¹ H NMR showed two singlets at 3.52 ppm (CH₃ OCF₂ CF₂ CF₂--) and 3.74 ppm ##STR26## in the ratio of 1/3.36 while the ¹⁹ F NMR wasconsistent with a mixture of ##STR27##

The remainder of the reaction product was heated at 100° for 4 hours andthen added to 50 ml water to give 4.5 g of product which contained 51%starting material and 38% product. An infrared spectrum of the materialcorresponding to the 38% product peak was identical to that of anauthentic sample of ##STR28##

Example 5 A. Preparation of CF₂ BrCFBrOCF₂ CF₂ CF₂ OCH₃

A 4.7 g mixture of CF₂ ═CFO(CF₂)₃ OCH₃ (ca. 70 mol %) and CClF₂ CCl₂ F(ca. 30 mol %) was reacted with excess bromine under irradiation of a GESun Lamp. The excess bromine was destroyed with aqueous sodiumbisulfite, and the product (lower layer, 4.2 g) was identified by gaschromatographic and NMR analyses to be CF₂ BrCFBrOCF₂ CF₂ CF₂ OCH₃containing a trace of CClF₂ CCl₂ F.

B. Preparation of CF₂ BrCFBrOCF₂ CF₂ COOCH₃

A mixture of 4.2 g CF₂ BrCFBrOCF₂ CF₂ CF₂ OCH₃ and 25 ml 96% sulfuricacid was heated at 120° for 4 hours and stirred at room temperature for16 hours. The reaction mixture was added to 200 ml cold water and thelower layer separated. The aqueous layer was extracted with CClF₂ CCl₂F, and gas chromatographic and NMR analysis showed the presence of CF₂BrCFBrOCF₂ CF₂ COOCH₃ and CClF₂ CCl₂ F as the only halogenatedcompounds.

C. Preparation of CF₂ ═CFOCF₂ CF₂ COOCH₃

When 29.2 g CF₂ BrCFBrOCF₂ CF₂ COOCH₃ in 5 ml tetraglyme was added to6.5 g zinc dust, 0.1 g iodine and 40 ml tetraglyme, an exothermicreaction was observed. The reaction mixture was distilled at 100 mm Hgpressure to give 15.1 g (84% yield) CF₂ ═CFOCF₂ CF₂ COOCH₃ whosestructure was confirmed by comparison of its infrared spectrum and gaschromatographic retention time with those of an authentic sample.

Example 6

Copolymerization of CH₃ OCF₂ CF₂ CF₂ OCF═CF₂ and Tetrafluoroethylene

A mixture of 17.0 g CH₃ OCF₂ CF₂ CF₂ OCF═CF₂, 35.0 g1,1,2-trifluoro-1,2,2-trichloroethane (F113), 0.02 g bis(4-t-butylcyclohexyl)peroxydicarbonate and 20 g tetrafluoroethylene washeated at 45° for one hour and 50° for three hours to give a polymericgel. The polymer, 6.5 g, was isolated by washing three times withmethanol and drying. The infrared spectrum of a thin film pressed at300° was consistent with a copolymer of tetrafluoroethylene and CH₃ OCF₂CF₂ CF₂ OCF═CF₂. The ¹⁹ F NMR spectrum of the copolymer was alsoconsistent with this structure and showed that the molar ratio of CF₂═CF₂ to CH₃ OCF₂ CF₂ CF₂ OCF═CF₂ was 5.06 to 1.00.

Example 7

Copolymerization of ##STR29## and Tetrafluoroethylene

A mixture of 10.7 g ##STR30## 20 g 1,1,2-trifluoro-1,2,2-trichloroethane(F113), 0.02 g bis (4-t-butylcyclohexyl)peroxydicarbonate and 15 gtetrafluoroethylene was heated at 45° for one hour and 50° for threehours to give a colorless gel. A white polymer, 1.8 g, was isolated bywashing three times with methanol and drying. The infrared spectrum of athin film pressed at 275° was consistent with a copolymer oftetrafluoroethylene and ##STR31## The ¹⁹ F NMR spectrum was alsoconsistent with this structure and showed the molar ratio oftetrafluoroethylene to ##STR32## to be 6.50 to 1.00.

Example 8

Hydrolysis of CF₂ ═CF₂ /CF₂ ═CFOCF₂ CF₂ CF₂ OCH₃ Copolymer and Use ofHydrolyzed Copolymer in a Chloralkali Cell

A mixture of 25 ml chlorosulfonic acid and 2.3 g of a copolymer of CF₂═CF₂ and CF₂ ═CFOCF₂ CF₂ CF₂ OCH₃ was stirred and heated for 5 hours at100°. The reaction mixture was carefully added to 500 ml ice and icewater and the polymer recovered by filtration. The polymer was heated in30 ml refluxing anhydrous methanol for 16 hours, filtered and dried. Athin film of the product could be pressed at 300° whose infraredspectrum was identical to that of a film of copolymer prepared by thecopolymerization of CF₂ ═CF₂ and CF₂ ═CFOCF₂ CF₂ COOCH₃.

A larger film of the material was pressed at 310°. It was hydrolyzed ina mixture of 300 ml water, 375 g methanol and 75 ml 10N sodium hydroxideat 60° for 66 hrs to give a film of the corresponding sodium salt 7.6mils in thickness. The film was mounted in a chloralkali cell andproduced 31.3% NaOH with a current efficiency of 94.2% at 4.34 volts.

I claim:
 1. A process for preparing a product copolymer comprising about70 to 95 mol % --CX₂ --CX₂ -- units wherein the four X's are fourfluorines or three fluorines and one chlorine, and about 5 to 30 mol %of substituted ethylene units of the formula ##STR33## wherein n is 0 or1, Z is F or OR', and R' is at least one member of the group consistingof CH₃, C₂ H₅ and H, from a starting copolymer comprising about 70 and95 mol % --CX₂ --CX₂ -- units and about 5 to 30 mol % of substitutedethylene units of the formula ##STR34## wherein X and n are as definedabove and R is CH₃ or C₂ H₅, said substituted ethylene units in each ofthe starting and product copolymers being randomly positioned throughoutthe copolymer chain, said process comprising treating said startingcopolymer with a strong acid at a temperature of at least 50° C. butbelow the decomposition temperature of said polymers and said strongacid.
 2. The process of claim 1 wherein Z is OR' and said strong acid isH₂ SO₄, ClSO₃ H, FSO₃ H or R_(f) SO₃ H wherein R_(f) is a perfluorinatedC₁ to C₈ group.
 3. The process of claim 2 wherein the four X's are fourfluorines, n is 0 and R is CH₃.
 4. The process of claim 2 wherein thefour X's are four fluorines, n is 1 and R is CH₃.
 5. A process forpreparing a copolymer comprising about 70 and 95 mol % --CX₂ --CX₂ --units wherein the four X's are four fluorines or three fluorines and onechlorine, and about 5 to 30 mol % of substituted ethylene units of theformula ##STR35## wherein n is 0 or 1, Z is F or OR', and R' is at leastone member of the group consisting of CH₃, C₂ H₅ and H, said substitutedethylene units being randomly positioned throughout the copolymer chain,said process comprising (1) copolymerizing CX₂ ═CX₂ with ##STR36##wherein X and n are as defined above and R is CH₃ or C₂ H₅, at atemperature of 0° to 200° C. and a pressure of 10⁵ to 2×10⁷ pascals inthe presence of a free radical initiator, and (2) treating the polymericproduct obtained in (1) with a strong acid at a temperature of at least50° C. but below the decomposition temperature of said polymericproduct, said copolymer and said strong acid.
 6. The process of claim 5wherein Z is OR' and said strong acid is H₂ SO₄, ClSO₃ H, FSO₃ H orR_(f) SO₃ H wherein R_(f) is a perfluorinated C₁ to C₈ group.
 7. Theprocess of claim 6 wherein the four X's are four fluorines, n is 0 and Ris CH₃.
 8. The process of claim 6 wherein the four X's are fourfluorines, n is 1 and R is CH₃.